1. Field of the Invention
This invention relates to an apparatus and method for supplying, mixing and dispensing a two-component bone cement.
2. Background Information
In many orthopedic surgical procedures, it is necessary to fix a prosthesis, such as an artificial joint, to living bone within a patient. Such procedures are used to treat osteoarthritis, rheumatoid arthritis, traumatic arthritis, avascular necrosis secondary to sickle cell anemia or collagen disease, severe joint destruction secondary to trauma or other conditions, and the revision of previous arthroplasty procedures. For such procedures, the use of a bone cement, in the form of an acrylic material comprising a reaction product of a methyimethacrylate monomer and a polymethyl-methacrylate-methylmethacrylate-styrene copolymer, is indicated. The use of such a bone cement is also indicated for the fixation of pathological fractures where loss of bone substance or recalcitrance of the fracture renders more conventional procedures ineffective. A bone cement of this type allows the seating and securing of a prosthesis to bone, and, when polymerization is complete, provides a buffer for the even distribution of weight and other stresses between the prosthesis and bone.
A bone cement of this type is available from Howmedica, a Division of Pfizer Hospital Products Group, Inc., Rutherford, N.J., under the registered trademark SURGICAL SIMPLEX.RTM. P, a radiopaque bone cement. This material is packaged as two separate, sterile components, which must be thoroughly mixed immediately before the cement is applied in surgery.
The first of these components is a colorless, flammable liquid, having a sweet, slightly acrid odor, supplied in an ampule containing a 20-ml full dose or a 10-ml half dose. This liquid component is composed of methyl methacrylate monomer, 97.4 percent by volume, N, N-dimethyl-p-toluidine, 2.6 percent by volume, and hydroquinone, 75.+-.15 parts per million. The hydroquinone is added to prevent premature polymerization, which may otherwise occur under certain conditions, such as exposure to light or elevated temperatures. The N, N-dimethyl-p-toluidine is added to promote the cold curing of the mixed compound. This liquid component has been sterilized by membrane filtration.
The second of these components is a finely divided white powder, supplied in a packet containing a 40-g full dose or a 20-g half dose. This powder component is composed of polymethyl methacrylate, 15 percent by weight, methyl methacrylate-styrene copolymer, 75 percent by weight, and barium sulfate, U.S.P, 10 percent by weight. The barium sulfate, U.S.P. is added to provide radiopacity. This powder component is sterilized by gamma irradiation.
When these liquid and powder components are mixed, an exothermic polymeric reaction forms a soft, pliable, dough-like mass. Within a few minutes, as this reaction progresses, a hard, cement-like complex is formed. These components may be manually mixed in a stainless steel or other inert container, into which the entire content of the powder component package is poured. The entire liquid content of the ampule is added to this powder, and the mixture is stirred with a sterile stainless steel spatula, or other suitable inert device, until the powder is completely saturated with the liquid. The mixture is then administered to the bone under pressure through a suitable sterile injection device, after stirring for 1 to 1 1/2 minutes in an operating room temperature of 75.degree. F.
Alternately, the mixture may be manually administered, after stirring is continued until a dough-like mass is formed. After a mixing and kneading process at least 4 minutes in duration, the fact that the mass does not stick to the rubber gloves of the operator provides an indication that the mass is ready for manipulation. The mixed and kneaded cement is applied to the bone manually or through the use of a suitable sterile pressurizing device, with the correct working consistency of the cement for application to bone being best determined by the experience of the surgeon.
Because the mixed cement sets so quickly, the two components are almost always mixed in the sterile environment of the operating room by a surgical assistant. A number of precautions must be followed. For example, the liquid component is a powerful lipid solvent, which has caused contact dermatitis in susceptible individuals. It is recommended that an individual working with these materials should wear two pairs of surgical gloves, while strictly adhering to established procedures, so that the compound dos not come into direct contact with sensitive tissues, and so that is not absorbed in his body. During the mixing of these materials, and during their chemical reaction, noxious and toxic vapors may be produced, which preclude, for example, the wearing of soft contact lenses in the operating room where the cement is being prepared. Although there is no present report indicating that the vapor is harmful, it is unpleasant, and it disturbs the breathing of many individuals. It is therefore particularly desirable to minimize the contact of operating room personnel with the vapor and to minimize the release of the vapor into the atmosphere.
Care must also be taken in the mixing of the liquid and powder components to ensure that the entire contents of the ampule and pouch are utilized. The mixing of these components should be thorough and vigorous. Data from in vitro studies have shown that monomer loss is related primarily to the frequency of stirring and secondarily to the duration of stirring. Adverse reactions affecting the cardiovascular system have been attributed to a leakage of unpolymerized liquid monomer into the circulatory system. On the other hand, caution should be taken to avoid kneading the cement too long, in order to avoid progression of the polymerization process to the point at which the cement is not adequately soft and pliable to obtain good filling of the bone cavities and fitting to the prosthesis.